Crystalline polyesters, particularly an aliphatic polyester and a polyalkylene terephthalate, have widely been employed since they exhibit excellent mechanical characteristics and heat resistance by crystallization. However, among polyester resins, a polylactic acid resin has a problem that it exhibits a low crystallization rate and therefore requires a crystallization treatment for a long time so as to obtain sufficiently crystallinity. Accordingly, various methods of improving the crystallinity have been studied (Patent Documents 1 to 4, etc.). However, any method is a method of adding a crystallization accelerator such as a nucleating agent and therefore has a problem that mechanical properties of the resin deteriorate when the additive amount of the crystallization accelerator is increased by enhancing the crystallinity.
In the prior art, there has been usually used a method in which a polyester resin in a molten state is pelletized by cooling using a water bath or the like for processing of the polyester resin. However, in this case, pellets in an amorphous state are obtained since the polyester resin is quenched from a molten state. When these pellets are subjected to a solid phase polymerization, pellets fuse to each other in the amorphous state, and thus requiring the step in which pellets are crystallized by performing a heat treatment or the like in advance. When the polyester resin has a low molecular weight, there occurs a problem that pellets are likely to fuse to each other in the crystallization step performed in advance since the polyester resin has a low softening point.